In recent years, optical discs including CD and DVD (Digital Versatile Disc) as typical examples thereof, and drives for recording and reproducing thereof are widely diffused in the world. Highly reliable storage stability is required for these discs, because they are used for storing data information, images, and the like. Further, with acceleration of price-reduction in discs, manufacturers of optical discs need to lower material costs but need to increase their productivity as well as to ensure the storage reliability of optical discs.
There are two types of optical discs, namely, recordable type and rewritable type. The former, recordable type optical discs generally use organic dyes as recording materials, and the latter, rewritable type optical discs generally use inorganic dyes as recording materials. For these optical discs, reflectance standards are provided so that reproduction of such optical discs is allowable with general-purpose drive players. A recordable type optical disc also is also required to keep a high reflectance, since the compatibility with drive players should be ensured. Materials each having a high reflectance is selected for the reflective layers, because recording layers for recordable type optical discs have properties of absorbing laser beam. As typical materials, there are Au, Ag, and Ag alloy. In this connection, in the case of a ROM disc (Read Only Memory disc), there is no layer for absorbing laser beam therein and a ROM disc offer ample room for the reflectance, and therefore, low-cost Al is used. Au and an Ag alloy respectively have (particularly Au has) a lower reflectance and are more costly than Ag but have excellent durability. An Ag alloy having a less dropped reflectance and good durability has been studied so far. Patent applications using an Ag alloy have been also filed, but such applications may not be employed in practical use because of bottlenecks of the reflectance. On the other hand, film-forming conditions for Ag reflective layers also exert influence upon recording properties. Higher-speed recording and higher-density recording causes increases in the influences.
For example, as described in Japanese Patent (JP-B) No. 3373626, if I (200)/I (111) is changed by changing sputtering conditions to change the film quality of an Ag reflective layer, this will affect recording properties. Then, the patent defines the film quality of the Ag reflective layer to have I (200)/I (111)≦0.2. Further, as described in Japanese Patent Application Laid-Open (JP-A) No. 2001-344825, it is found that changing the quality of Ag film would affect storage reliability of optical recording media. The patent application defines the film quality of the Ag reflective layer to have I (200)/I (111)>0.4.
However, although what is desired for an Ag reflective layer is to have excellent storage properties and is never having negative impacts on recording quality, it is found that Ag reflective layers formed within the ranges of film quality of the above-mentioned patents had negative impacts on either storage properties or recording properties.
In forming recording pits, when introducing laser beam into a disc, the recording layer of the disc absorbs the laser beam to change it to heat, and then recording pits are formed by thermal decomposition, and the way heat diffuses at that time has greatly impacts on recording properties. By changing sputtering conditions for film-forming to form reflective layers, each surface formation of the reflective layers will vary. At the same time, a change in the condition of the reflective layer adjacent to the recording layer will also affect the thermal conductivity. Thus, film-forming conditions of reflective layers have significant impacts on recording properties.
Especially in the case of high-speed recording, the way heat diffuses is very important, because the quick incident beam having a large amount of power is introduced.